The concept of uncertainty is hard to grasp. In my high school (1979) we were required to use slide rules for the first year of science class, despite the availability of inexpensive calculators. The reason was to impress upon us the importance of significant figures, and also to develop our intuition for orders of magnitude. Unless your slide rule is really expensive, you just can't get more than 3 significant figures from it, which is a good match to the accuracy of just about every measurement in high school. It also tells you the figures without the magnitude--you have to determine in your head whether you just calculated microns or kilometers.